The present invention relates to a semiconductor integrated circuit, and in particular to a single chip microcomputer (hereafter referred to simply as a microcomputer). Further, the present invention relates to a semiconductor integrated circuit suitable for significantly reducing the number of components constituting a system and suitable for shortening the development interval of an application system.
In apparatus of the prior art, a microcomputer and a PLA (programmable logic array) are fabricated to form one chip, and the PLA functions in response to an external input irrespective of the program of the microcomputer. Outputs of the PLA then activate various functions and the microcomputer is started by the result of these functions.
The PLA is a logic circuit apparatus having a variable logic structure, the logic function of which can be arbitrarily constructed using an electrical write operation to a non-volatile semiconductor memory device.
On the other hand, an improved PLA using the EPROM technique is disclosed in U.S. Pat. No. 4,609,986.
Further, a semiconductor integrated circuit apparatus having a data processing unit, such as a CPU and a non-volatile memory block for storing therein software in an identical chip, has been disclosed in JP-A-60-198667.
On the other hand, electrically writable logic devices are discussed in the products catalogue of Altera Corporation, p. 12 or Electronic Design, Aug. 7, 1986 pp. 94 to 97.
The prior art apparatus disclosed in the aforementioned JP-A-61-285567 has the effect of reducing the number of components in some applications. However, the applicability of this prior art apparatus to general use is not available, and hence a wide range of applications cannot be covered, resulting in a problem. Further, the aforementioned U.S. Pat. No. 4,609,986 discloses only the PLA itself. This patent discloses only provision of a desired single chip microcomputer to the user in a short delivery time achieved by integrating the PLL, i.e., integrating the programmable logic circuit and the microcomputer into one chip, by constituting a memory block for storing therein a program, which constitutes the software of the single chip microcomputer, in the form of a non-volatile memory, and by writing the program into the non-volatile memory for storing the program from the outside of the chip under the directive of the user itself. Ways of allowing the user to freely construct a logic apparatus for implementing the hardware of a single chip microcomputer have not been considered.
The software (program) of the single chip microcomputer is developed, and the software thus developed is written into the ROM (read only memory) of the memory block of the single chip microcomputer. If this ROM is composed of a non-volatile memory and the user freely writes the program into this nonvolatile memory from the outside of the chip, the user can obtain a single chip microcomputer containing the desired software in a short time.
As for the hardware of the single chip microcomputer, respective users often specify various requirements. Therefore, it is not possible to satisfy simultaneously various requirements of the user with unified hardware. In the prior art, therefore, a random logic circuit or the like is added as an external circuit of the single chip microcomputer in order to realize requirements of respective users, resulting in problems involving the need for a large number of components used in the overall system and a large mounting area. A basic configuration of a single chip microcomputer for solving this problem is already disclosed in JP-B-63-91563 (U.S. Pat. application Ser. No. 238,534, filed on Aug. 31, 1988 in United States now abandoned).
As examples of the hardware of a single chip microcomputer for which respective users have their peculiar specifications, various examples such as a key input encoder (which is an encoder for converting information inputted from a keyboard and for taking the resultant information into the single chip microcomputer), a display decoder (a decoder for converting information outputted from the single chip microcomputer and for driving a display unit), a time measuring timer and a serial communication circuit can be mentioned.